1. Field of the Invention
This invention relates to a surface-acoustic-wave device which can function as a surface-acoustic-wave amplifier, and more particularly to a surface-acoustic-wave device having a depletion layer control means formed of a p-n junction at an interface portion between a semiconductor and a piezoelectric layer which is capable of preventing application of a DC voltage to the piezoelectric layer so as to minimize voltage hysteresis and assure stable characteristics.
2. Description of Prior Art
As disclosed in U.S. Patent Application Ser. No. 948,826, the assignee of the present invention has developed a surface-acoustic-wave device as illustrated in FIG. 1, which is operative by a continuous wave and has a desired S/N ratio, a high Q value when used as a frequency selecting device, and a wide available input frequency band.
In FIG. 1, 1 is a semiconductor substrate, on which an insulator film 2 and a piezoelectric layer 3 are superposed to form a laminate, and 4 is an electrode provided on the piezoelectric layer 3 for applying a DC bias voltage and a pumping power thereto. 5 is an ohmic electrode, 6 is a DC power source for applying a DC bias voltage, and 7 is a high frequency power source for applying a pumping power.
In the so formed surface-acoustic-wave device, it will be seen that when a DC bias voltage and a pumping power are applied to the laminate through the electrode 4, a surface-acoustic-wave input signal 8 is amplified, in the course of propagation through an operation region beneath the electrode 4, by a parametric interaction with a depletion layer capacitance at a surface of the semiconductor substrate 1 through a piezoelectric effect of the piezoelectric layer 3, and amplified surface-acoustic-wave signals 9, 9' are outputted through suitable output means (not illustrated).
In the practical manufacturing of the surface-acoustic-wave device as described above, the following materials may be selected for the respective components of the device to curtail the material costs and the manufacturing costs: silicon (Si) for the semiconductor substrate 1; silicon dioxide (SiO.sub.2) for the insulating film 2; and zinc oxide (ZnO) for the piezoelectric layer 3. The piezoelectric layer 3 of zinc oxide is easily laminated on the substrate 1 by an ordinary method such as evaporation, sputtering, etc.
As described above, for operation, this conventional surface-acoustic-wave device is applied, at the surface of the semiconductor substrate 1, with a DC bias voltage through the electrode 4. In this connection, it is to be noted that there is caused a phenomenon of voltage hysteresis, after the DC bias voltage is removed (the voltage is lowered to zero). The voltage hysteresis is a phenomenon that the state of the surface portion of the semiconductor substrate 1, once applied with the DC bias voltage, does not return to its original state after removal of the voltage. Therefore, it is difficult to definitely determine a value of a bias voltage for an optimum operation and to assure stable characteristics.
Although a cause of the phenomenon is not yet fully known, it has been said that electric charge injected into the layer of zinc oxide formed by evaporation through the electrode 4 by application of the DC bias voltage is caught by the layer and moves through the layer.